Conventional disk drives employ various reading and writing methods. In the magnetic memory method, a memory section formed by a magnetic powder is magnetized either longitudinally within its plane or perpendicular to the plane of the memory section. Another type of prior art apparatus employs an opto-magnetic recording method in which a magnetic film is magnetized in a predetermined direction and a laser beam is then applied to a position on the film where recording is to be effected to raise the temperature at the recording position, thereby inverting the direction of magnetization. Another type of prior art employs an optical recording method wherein a film is written by a focused laser beam which heats up and deforms the film, and is read by a lower power laser beam. Another type of prior art operating on a molecular scale employs a fine tip to change the state of electrons on a variety of different surfaces.
The conventional disk drives that employ the above described methods suffer, however, from the following problems. Magnetic memory systems employ a read/write mechanism which can come in contact with the memory section. Over time this degrades the operation of the memory system. The size of the memory domains are dependent on the size of the read/write head. The read/write head is difficult to manufacture in order to achieve small domain sizes. Also, as the domain size gets smaller, it becomes necessary for the read/write head to operate closer to the memory surface. This increases the problems of manufacturing both the head and the memory surface for flatness. It also increases memory and read/write head wear, reducing the lifetime usefulness of the memory device. Compact disk optical memory systems are a read-only type of memory. This limits their usefulness is applications which require that data be changed and updated often.
Many optical memory systems can have data written only once. After the entire memory area has been written, the device becomes a read-only memory, limiting its usefulness in applications which require that data be changed often. Optical systems are limited by the wavelength of the light used in the reading and writing apparatus, and the size of the optical system guiding the light to the memory surface. Diffraction limiting requires that, for a smaller domain size, the optical system must get closer to the memory surface. This increases head wear and reduces the lifetime of the memory device. These limits define the minimum size of the memory .domain to a value much greater than the size of a single atom or molecule.
In U.S. Pat. Nos. 4,945,515 & 4,962,480 of Ooumi et al., molecular memory systems operating via the changing of states of electrons are described. These systems are limited by the low mass and high mobility of electrons. The electron charge state that is written in the memory rapidly diffuses, causing a rapid loss of memory data. The patents also describe using multiple needles that are linked together and must move as a group. This limits the practical applicability of this arrangement, since molecular surfaces are rarely flat over a large area. Also, the need to manufacture the tips to conform to the surface within the few Angstroms needed for the tunneling effect is a limiting factor.
In U.S. Pat. No. 5,038,322 of van Loenen, a system for inscribing information into a molecular memory surface is described. This system requires a fine tip to be raised and lowered into the memory surface, and the tip must come in contact with the memory surface. The tip must be raised and lowered for each bit of information. The mass of the tip and the force driving the tip defines the speed at which this process can operate. These parameters require that this speed be limited to far below the Megahertz rate at which typical disk memory systems operate. Also the need for the tip to contact the surface greatly increases the wear on both the tip and the memory, reducing the lifetime of the device.
In U.S. Pat. No. 5,051,977, Goldberg describes a molecular memory system utilizing a fine tip for writing and a modulated focused light source to create a chemical modification of a molecular memory surface to read the memory. A fluorescence property of an underlying substrate in conjunction with the altered tunneling properties of the information layer is used to read the information layer as the fine tip is passed over the information layer. This system also suffers from being a write once memory system, as the chemical process induced by the tip and light source is not reversible. In addition, the additional complexity of an optical system for writing and reading the memory increases the cost of the system.
In U.S. Pat. No. 5,091,880, Iseno et al. describe a molecular memory system utilizing a fine tip for reading and for writing and a focused light source and light sensor for determining position. As this system uses a conventional optical system for finding the track and sector information, the number of tracks, and consequently the overall density, is limited by diffraction of the light focused on the memory surface as in optical memory systems. Also, the additional complexity of the optical system increases the cost of the system.
In U.S. Pat. Nos. 4,826,732 and 4,829,507 of Kazan et al., a molecular memory system is described which utilizes the adsorption or de-adsorption of oxygen, sulphur, or gold atoms on an adsorbent carrier. This process uses a monatomic layer of oxygen, sulphur, or gold on a bulk substrate of zinc oxide (ZnO), zinc indium sulphide (ZnIn.sub.2 S.sub.4), indium oxide (In.sub.2 O.sub.3), titanium dioxide (TiO.sub.2), cadmium sulphide (CdS), or silicon. In addition the process of pre-reading topological information is described, which requires the scanning of the entire usable memory surface. The very large number of possible tracks on a molecular disk requires a prohibitively long amount of time to pre-read the topology. The use of oxygen as a memory element requires that the system be operated in a vacuum, or a nitrogen or other inert (non-air) atmosphere, requiting expensive seals and environment maintenance systems. Although Kazan et al. describes a method for de-adsorbing oxygen so as to require a controlled environment, it is likely that, over a relatively short time, oxygen atoms from the air will re-attach to the substrate, destroying the integrity of the stored data.
In U.S. Pat. No. 4,575,822, Quate describes a molecular memory system composed of a substrate with physical perturbations, a substrate with magnetic domains, a substrate with two layers such that an electric charge is stored, or a substrate composed of a superconducting film, all readable via tunneling current and a fine tip. Superconducting films in current technology must still be cooled far below room temperature, requiting expensive cooling apparatus, and would loose information should the cooling apparatus fail. Using electric charges as the data storage element suffers from the problem of the low mass of the electrons, and the subsequent rapid diffusion of charge, causing loss of stored data, as described above. The portion of the disclosure relating to physical perturbations describes the perturbations as being created by disruption of the surface by a physical probe, focused laser beam, electron beam or other radiation or particle beam. A physical probe suffers from the problem of having a large, irregular surface relative to atomic scale. Attempts to push the tip into the substrate require the need for rapid up and down movements in the Z axis. The speed of this depends on the mass of the tip being moved, and it is difficult to manufacture such a tip mechanism small enough to allow asynchronous up and down movement in the Megahertz range that is required for a disk drive. This up and down hammering would also deform the tip surface in a short time, rendering the tip useless. A focused laser beam suffers from the same size limits as other optical systems as described above. Electron or particle beams also suffer from this focusing problem. The apparatus described in the Quate patent also would probably have to pre-read the topology of the substrate, although this is not mentioned in the patent.
In U.S. Pat. No. 4,916,688, Foster et al. describe a molecular memory system composed of a film of material whose state is altered by controlled heating of local spots in the film surface by a current through a fine tip. This system suffers from the amount of time required to form, and particularly to erase, the spots on the film. The time to erase is much longer by orders of magnitude than the time to write, and requires a careful control of voltage and current levels. Also, during this time, there is no way to measure the position of the tip above the surface. When the tip is in heating mode the current levels between the tip and the surface are much higher than when in tunneling mode. The tip also must come in contact with the surface to provide a sufficient highly conductive path for the heating process to occur. This induces wear on the tip.